In gas turbine engines, disks which support turbine blades rotate at high speeds in an elevated temperature environment. Increased engine efficiency and engine performance require advanced gas turbine engines to operate at ever higher temperatures. The temperatures encountered by the disks of these engines at their outer or rim portion may be 1500.degree. F. or higher, while the temperatures at the inner or hub portion will typically be lower, e.g. of the order of 1000.degree. F. The different operating conditions and temperatures to which the separate portions of the disks are exposed call for different combinations of mechanical properties. The high temperature rim portion must have time dependent or hold time fatigue crack growth resistance and creep resistance, while the highly stressed hub portion must have high burst strength at relatively moderate temperatures and fatigue crack growth resistance. The hub portion also must have high resistance to low cycle fatigue to ensure long component life.
Because of these differing requirements concerning the mechanical properties of the separate disk portions, and the extreme temperature gradients along the radius of a turbine disk, it is difficult, if not impossible, for a single alloy to satisfy the requirements of both the hub and the rim area of a turbine disk of the type that is used in an engine of advanced design. For example, many forged nickel-base alloys have superior tensile and low cycle fatigue properties, but quite limited creep rupture strength or hold time fatigue crack growth resistance, while other nickel-base alloys have excellent creep rupture strength, but poor tensile and fatigue properties.
One solution for meeting the higher operating temperatures required in these more efficient and more powerful advanced engines is to increase the weight of the disk to reduce stress levels, when the alloy used is metallurgically stable and not prone to damage at the desired high temperature. This solution is unsatisfactory for aircraft due to the undesirable increase in the weight of the system which negates the advantages of increased power and efficiency.
Another solution is to use a dual alloy disk wherein different alloys are used in the different portions of the disk, depending upon the properties desired. The disk has a joint region in which the different alloys are joined together to form an integral article. Numerous means for fabricating dual alloy disks have been suggested or evaluated. As employed herein, the term joint refers to a metallurgical joint wherein the joined members are held together by the fusion of their metals or with a third metal as in the case of a diffusion braze or diffusion weld, as opposed to a mechanical joint wherein the joined members are held in contact by mechanical means such as bolts or rivets. The joint and region of altered metal adjacent thereto are referred to as the joint region.
Although fusion welding has been suggested as a solution, the nickel-base superalloys of the type used in disks are difficult to weld in the required configuration.
Inertia welding is a possible alternative. However, with very dissimilar alloys, there is a potential for uneven flow, inadequate joint clean-up and incipient melting in the heat-affected zone. This process also requires large equipment and specialized tooling.
Another technique for bonding parts made of different alloys is by diffusion bonding, as applied to nickel-base alloys. However, this method is currently considered not sufficiently reliable for producing dual alloy disks.
Another method is referred to as bicasting, or casting one portion of an article, such as a rim, directly against another portion, such as a wrought or a forged hub. This method provides an unacceptable mechanical joint, as distinguished from a metallurgical joint. Further, the fact that one portion of the article is necessarily cast causes at least that portion to have all of the potentially inherent defects of a casting, such as inhomogeneities, shrinkage, inclusions and porosity. The presence of such defects is unacceptable for a high speed rotating part.
Still another fabricating technique is hot isostatic pressing. This technique may be employed to consolidate powder used for one portion of a disk, such as the hub, and also to join it to the other portion. In a variation of hot isostatic pressing of powders, two wrought sections are welded together in a hot isostatic press. This technique requires a gas-tight enclosure, such as a separate can, a weld or a braze, around the exposed sides of the joint regions. In yet another variation of the hot isostatic pressing method, an annular ring of powder is hot isostatically pressed between two wrought members. However, the disadvantage of hot isostatic pressing is that any impurities present at the joint prior to hot isostatic pressing will remain there.
Billets made by coextrusion and isoforging, in which a core is made from one alloy and an outer portion is made from another alloy, have been manufactured with relatively little difficulty. However, much additional research is needed to develop forging procedures to control the precise location and shape of the interface between the alloys.
Explosive welding has been used to weld combinations of dissimilar alloys. This process has been found to be useful for cladding one alloy onto the surface of another. Such a process is, however, presently unuseable for joining dual alloy disks, in that the configuration of the joint region of such disks is not suitable for the introduction of explosive energy for bonding a hub to a rim.
Another approach has been to make a single alloy disk whose different parts have different properties. U.S. Pat. No. 4,608,094 which issued Aug. 26, 1986, outlines a method for producing such a turbine disk. The disk is made from a single alloy which has been worked differently in different regions to yield different mechanical properties. Such a disk is, however, subject to the limitations of the single alloy employed. Alternatively, a single alloy disk may have different portions subjected to heat treatment at different temperatures, or at the same temperatures for different times. Such a differential heat treatment will produce a disk having different mechanical properties in different portions. However, the disk is still subject to the previously mentioned limitations of the single alloy used.
U.S. Pat. No. 3,940,268 which issued Feb. 24, 1976, provides a solution for turbine disk/blade assemblies. It discloses a disk of powdered metal material connected to a plurality of radially, outwardly-directed airfoil components located in a mold and metallurgically bonded during hot isostatic formation of the disk element. While blades can be bonded to a disk of a different material by the method set forth in the '268 patent, hybrid or composite turbine rotor structures formed by such methods lack precision and dimensional control between adjacent airfoil components. Such control is required to maintain the desired gas flow through adjacent passages of the airfoil components connected to the disk. Additionally, this solution addresses problems of joining blades to a disk, and not the problems of joining a hub to a rim to form a disk.
The concept of forming a rim portion of a disk with a coarse grain and a central portion of a disk with a fine grain is disclosed in NASA Report No. CR-165224 entitled "Development of Materials and Process Technology for Dual Alloy Disks". The report indicates that the rim portion of a disk is formed from powdered metal by hot isostatic pressing of powdered metal. The hub portion of the disk is then filled with metal powder and is enclosed in a container. The enclosed rim portion and the powdered metal are then subjected to a hot isostatic pressing operation to produce a dual alloy turbine disk.